Generally speaking, a plasma torch, such as the one mentioned in the document U.S. Pat. No. 3,301,995, includes two coaxial tubular electrodes with one in prolongation of the other, each of the electrodes being disposed in a support surrounding it. The plasma torch comprises means to produce the initiating of an electric arc between the two electrodes, as well as means to inject plasmagene gas, such as air, into a chamber between said electrodes. In addition, electrode cooling means are provided in each electrode support. Advantageously, the plasma torch is provided with means to move the catching feet of the electric arc so as to avoid any premature wear of the internal surfaces of the tubular electrodes. In order to achieve this, these means include at least one electromagnetic coil surrounding one of the electrode supports and, by modulating the axial magnetic field delivered by the coil, the catching feet of the electric arc thus move along said internal surfaces of the electrodes.
As regards initiating of the electric arc, there are basically two distinct methods.
In the first method, a high voltage (several tens of kilovolts) is applied to the terminals of the electrodes, which generates a discharge (disruptive breakdown) between the two electrodes and produces the electric arc. This solution requires that a short distance needs be provided between the opposite faces of the electrodes and thus complicates the arrangement of the chamber for injecting the plasmagene gas. In addition, an auxiliary power unit is necessary.
According to the other method, a temporary short-circuit is effected between the electrodes by means of an auxiliary, expansible and retractable initiating electrode, such as the one described in the documents FR-Pat. No. 2 479 587 and U.S. Pat. No. 3,301,995. This method requires that a complex mechanical device be provided outside the torch so as to actuate the initiating electrode which requires an additional external spatial requirement. However, this initiating method by short-circuit is more reliable and less expensive than the initiating method by discharge.